<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 200px;
            height: 200px;
            background-color: skyblue;
            /* border: solid实线 dashed虚线 dotted点线 double双线
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            */
            /* border: 2px soild black ; */
            /* border: 2px dashed black ; */
            /* border: 2px dotted black ; */
            border: 2px double black ;
        }
    </style>
</head>
<body>
    <div>testtest</div>
</body>
</html>